JP 7-74382 discloses a method for producing hot-rolled steels for body-reinforcing electric resistance welded tubes by induction hardening. The technique disclosed in JP 7-74382 can produce body-reinforcing electric resistance welded tubes required to have high strength, such as door guard beams and bumper reinforcements, which have high energy absorbability due to induction hardening. However, that technique has the problem of failing to achieve formability, fatigue endurance after quenching, and low temperature toughness required for suspension and chassis members.
JP 2000-248338 discloses steel sheets for induction hardening and induction-hardened reinforcing members, which have excellent hardenability, hardened portions with toughness, and excellent high energy absorbability, and a method for producing these. The technique disclosed in JP 2000-248338 can produce body-reinforcing members, such as center pillars and bumper reinforcements, which have excellent high impact energy absorbability due to induction hardening. However, that technique has the problem of failing to achieve fatigue endurance after quenching and low temperature toughness which are required for suspension and chassis members.
JP 2605171 discloses high toughness electric resistance welded tubes for heat treatment which can impart high strength and toughness by heat treatment. The technique disclosed in JP 2605171 can produce automobile door reinforcements made from steel tubes by, for example, quenching after induction heating, the reinforcements having high strength and excellent low temperature toughness. However, that technique has the problem of failing to achieve fatigue endurance, resistance for hydrogen embrittlement, and corrosion fatigue endurance required for suspension and chassis members.
JP 2000-248331 discloses low alloyed steel sheets having excellent properties of heat treatment by irradiation of a high energy density beam, high fatigue endurance after quenching, and high workability. The technique disclosed in JP 2000-248331 can improve local fatigue endurance. However, that technique has the problem of failing to secure fatigue endurance required over the wholes of suspension and chassis members and to achieve resistance for hydrogen embrittlement and corrosion fatigue endurance required for these members.
WO 02/070767 A1 discloses electric resistance welded tubes for hollow stabilizers which have excellent workability. The technique disclosed in WO 02/070767 A1 can produce electric resistance welded tubes for hollow stabilizers having homogeneous metal structures in electric resistance welded portions and matrix portions and excellent workability due to induction heating and diameter reduction rolling of the electric resistance welded tubes. However, that technique has the problem of failing to achieve fatigue endurance, resistance for hydrogen embrittlement, and corrosion fatigue endurance required for suspension and chassis members.
JP 3111861, JP 3374659 and JP 2003-138316 disclose high tension electric resistance welded tubes which have excellent resistance for hydrogen embrittlement. The techniques include increasing tensile strength of steel strips and then forming tubes to produce high tension steel tubes having excellent resistance for hydrogen embrittlement. However, those techniques have the problem of failing to achieve formability and fatigue endurance required for suspension and chassis members.
It would therefore be advantageous to provide a steel having excellent formability, excellent fatigue endurance after quenching, excellent low temperature toughness, excellent resistance for hydrogen embrittlement, and excellent corrosion fatigue endurance for suspension and chassis members, and a technique for producing the steel.